In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipments (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served b a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC) which supervises and coordinates various activities of the plural base stations connected thereto.
The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. The 3GPP has developed specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN). The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Access Gateways, or AGWs) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
3GPP Release 10 provides for a LTE random access procedure which is used in several situations: for initial access when establishing a radio link (moving from Radio Resource Control (RRC)_IDLE to RRC_CONNECTED state); to re-establish a radio link after radio-link failure; to establish uplink synchronization; or, as a scheduling request if no dedicated scheduling-request resources have been configured on the Physical Uplink Control Channel (PUCCH). The 3GPP Release 10 LTE random access procedure essentially comprises four basic steps which encompass a sequence of messages exchanged between the terminal and the eNodeB, as generally illustrated in FIG. 1. In FIG. 1, the four steps essentially correspond to the solid arrows, whereas the dotted arrows essentially correspond to control signaling for the solid arrow step which the dotted arrows precede. For example, the second step is the second arrow (dotted) and the third arrow (solid). The second arrow (dotted) tells the UE to listen to the third arrow corresponding to the second step. In the same way the fourth arrow tells the UE to transmit the fifth arrow corresponding to the third step. Further in the same way the sixth arrow tells the UE to listen to the fourth step in the RA-proceedure corresponding to the last arrow. These basic four steps are briefly discussed below.
A first step in the random-access procedure comprises transmission of a random-access preamble on the Physical Random-Access Channel (PRACH). As part of the first step of the random-access procedure, the terminal randomly selects one preamble to transmit, out of one of the two subsets defined for contention-based access as illustrated in FIG. 2. Which subset to select the preamble from, is given by the amount of data the terminal would like to (and from a power perspective can) transmit on the UL-SCH in the third random access step. A time/frequency resource to be used for these transmissions is illustrated in FIG. 3, which is understood by reading “4G-LTE/LTE Advanced for Mobile Broadband” by E. Dahlman et al, Academic Press, 2011, incorporated herein by reference. The time/frequency resource to be used is given by the common PRACH configuration of the cell, which can be further limited by an optional, UE specific mask, which limiting the available PRACH opportunities for the given UE. This is more thorough described in “3GPP TS 36.321 v.10.0.0. Medium Access Control (MAC) protocolspecification” and “3GPP TS 36.331 v.10.3.0.Radio Resource Control (RRC) protocol specification”, both of which are incorporated herein by reference.
A second stop of the random access procedure comprises the Random Access Response. In the Random Access Response the eNodeB transmits a message on the DL-SCH containing the index of the random-access preamble sequences the network detected and for which the response is valid; the timing correction calculated by the random-access preamble receiver; a scheduling grant; as well as a temporary identify (TC-RNTI) used for further communication between the UE and network. A UE which does not receive any Random Access Response in response to its initial random-access preamble transmission of step 1 above within a pre-defined time window, will have considered the attempt failed, and will repeat it random access pre-amble transmission (possibly with higher transmit power) up to a number of maximum of four times, before considering the entire random-access procedure failed.
The third step of the random access procedure serves, e.g., to assign a unique identity to the UE within the cell (C-RNTI). In this third step, the UE transmits the necessary information to the eNodeB using the UL-SCH resources assigned to the UE in the Random Access Response. This message, also known as the RRC Connection Request message allows the UE to adjust the grant size and modulation scheme as well as allows for HARQ with soft combining for the uplink message.
The fourth and last step of the random-access procedure comprises a downlink message for contention resolution. The message of this fourth step is also known as the RRC Connection Setup message. Based on the contention resolution message each terminal receiving the downlink message will compare the identity in the message with identity transmitted in the third step. Only a terminal which observes a match between the identity received in the fourth step and the identity transmitted as part of the first step will declare the random-access procedure successful, otherwise the terminal will need to restart the random access procedure.
The UE power to use in the random access attempt is calculated according to a specified formula, known from “3GPP TS 36.213 v.10.6.0. Physical layer procedures”, reproduced as Expression 1 below, with parameters carried in the system information. If the UE does not receive a RandomAccessResponse in the second step of the procedure, the transmit power of the following PRACH transmission is increased by a parameter delta value up until limited by the UE maximum power:PPRACH=min{PCMAX,c(i),PREAMBLE_RECEIVED_TARGET_POWER+PLc}_[dBm]  Expression 1:
In Expression 1, PCMAX,c(i) is the configured UE transmitting power as defined in “3GPP TS 36.213 v.10.6.0. Physical layer procedures” for subframei of the primary cell and PLc is the downlink pathloss estimate calculated in the UE for the primary cell.
As currently being discussed in 3GPP Coverage Enhancements TR 36.824, incorporated herein by reference, there may be situations where a UE is unable to access the network due to RACH coverage problems, e.g., the UE may have BCCH coverage and can measure on the cell and read the cell's system information, but the network cannot not receive any random access preamble attempts from the UE because the UE is power/coverage limited, and hence the received signal in the network is thus too weak. This may be the case, for example, for a user placed indoor served by a cell with high output power.